In my previous U.S. Pat. No. 4,455,870, issued on June 26, 1984, a means is provided to measure flow through a pumping station utilizing constant speed pumps which cycle on and off. Typically, a pump will start pumping when the wet well reaches a "high" level, and will cease pumping when the wet well reaches a "low" level.
There are pumping stations where the system set forth in my above U.S. patent cannot be used, because the pumping stations utilize variable speed pumps. In such systems, so long as sewage is coming into the pumping station, at least one pump is running, although this may be at a low rate of speed. In stations utilizing variable speed pumps, the aim is to get the sewage or water moving out of the station at about the same rate as it comes in. This is accomplished by continuously adjusting the speed of the pump or pumps so as to maintain a given level in the wet well.
In these systems, the method for level detection is usually either ultrasonic or the bubbler type (based on pressure). As the level in the wet well tends to rise, the speed of the pump is increased, and this is normally accomplished by using a frequency control device. Conversely, when the level in the wet well goes down, the speed of the pump decreases.
Conventionally, liquid flow through a pumping station with a variable speed pump or pumps is measured by a magnetic flow meter or an open channel flow meter at the inlet to the wet well. However, these methods leave much to be desired in terms of accuracy.